Destiny's classes hew close to the holy RPG trinity of Fighter, Rogue and Wizard. Titans are the Fighter analogue, wearing thick, bulky armor and described on the Destiny site as "steadfast and sure." Hunters are the Rogue-like class of Destiny, possessing a "unique brand of daring and ingenuity." Lastly, Warlocks align with the Wizard archetype, using "arcane energies" to slay their foes.
The four-armed Fallen are described as hardy warriors who use guerrilla tactics to hit their enemies and immediately disappear. There's also the ghoulish Hive, the massive and militaristic Cabal, and the robotic Vex. You'll be able to take on all of the above across the surfaces of Earth, Venus, Mars and Luna (that'd be the proper name of our moon - the more you know).